1. Field of the Invention
This invention relates to a curable resin composition. More particularly, this invention relates to a curable resin composition containing uniformly dispersed and finely divided particles of a silicone rubber. The composition exhibits improved flow properties during molding, and converts to a cured material exhibiting excellent flexibility, a low thermal expansion coefficient, a low mold shrinkage ratio and excellent adhesion between the resin matrix and dispersed silicone rubber particles.
2. Description of the Prior Art
Cured compositions based on organic and silicone resins have excellent electrical properties, including dielectric properties, volume resistivity, dielectric breakdown strength, and excellent mechanical properties that include flexural strength, compression strength, and impact strength. These properties make the compositions particularly desirable for use as insulating materials for various types of electric and electronic components.
The curable resin compositions can be fabricated by transfer molding, injection molding, potting and casting. The compositions can be applied to substrates by powder coating, immersion coating or dipping.
Cured resins prepared using the aforementioned compositions are generally rigid. When these resins are used, for example, to seal an electric or electronic component, large mechanical stresses are imparted to the internal elements of the component during the heating required to seal the component and post-cure the resin or the thermal cycling to which the component is subjected during testing and use. As a consequence, the element may not function properly or failure may occur in part of the element. One cause of the stresses and resultant failures is the difference in thermal expansion coefficient and post-molding shrinkage ratio between the elements of electric and electronic components and curable resins. The elements of electric and electronic components have very low thermal expansion coefficients and shrinkage ratios while the resins have large values for these ratios.
These differences between the thermal expansion coefficients and shrinkage ratios of the resin and the coated component also result in the formation of cracks in the cured resin coating and gaps between the component and the resin. The infiltration of water and other undesirable materials into these gaps contributes to deterioration and subsequent failure of the elements comprising the components.
Most previous attempts to modify curable resin compositions have not had as their objective a reduction in the thermal expansion coefficient or post-molding shrinkage ratio of the cured resins. For example, Japanese Patent Publication Number 13241/49, published on Dec. 18, 1974 relates to an improvement in the lubricating properties of the surfaces of resin moldings achieved by the addition of an organosilsesquioxane powder to curable phenolic resin compositions.
Japanese Patent Application Laid Open [Kokai] Number 52-14643 [14,643/77], published on Feb. 3, 1977, relates to an improvement resistance of silicone resin to abrasion by metals. This improvement is obtained by filling the synthetic curable resin with finely divided particles obtained from a cured material based on a organopolysiloxane and an inorganic filler. The thermal expansion coefficient, post-molding shrinkage ratio, and flexural modulus are unsatisfactory in both of the aforementioned resin compositions following curing.
In Japanese Laid Open Patent Application Number 58-219218 [219,218/83], published on Dec. 20, 1985 the present inventors propose a solution to the problems described in the preceding paragraph by adding to the curable resin a cured material containing at least 10 weight % of a straight-chain siloxane fraction. The material is reduced to a finely divided, i.e. microparticulate, form following curing. However, the problem with this approach that it is not always easy to reduce an elastomeric cured material to the desired small particle size.
The present inventors in Japanese Laid Open Patent Application Number 59-96122 [96,122/84], published on June 2, 1984 disclose preparing spherical cured particles by spraying a curable elastomer composition into a current of heated air. This method is quite excellent although expensive, due to the cost of the equipment required to produce the spherical particles of cured rubber.
U.S. Pat. No. 4,743,670, which issued to K. Yoshida et al. on May 10, 1988 describes a method for preparing a powdered form of silicone rubber by forming a dispersion of a liquid silicone rubber in surfactant-free water at a temperature of from 0 to 25 degrees C. and then mixing this dispersion into water at a temperature greater than 25 degrees C. Organic resin compositions containing these particles are less than satisfactory with respect to certain properties, particularly the infiltration of moisture into the cured resin. U.S. Pat. No. 4,778,860, which issued to two of the present three inventors on Oct. 18, 1988 attempts to solve these problems, by adding to the curable silicone rubber composition an aromatic hydrocarbon compound containing a substituted benzene ring where the substituent is a monovalent ethylenically unsaturated hydrocarbon radical or an alkenyloxy radical. This approach is not totally successful.
The present inventors have now discovered that the dimensional stability and resistance to moisture infiltration exhibited by the resin compositions disclosed in the aforementioned U.S. Pat. No. 4,778,860 could be increased to acceptable levels by adding an ethylenically unsaturated epoxy compound and an ethylenically unsaturated aromatic compound to the curable rubber composition.
Accordingly, an objective of the present invention is to provide a curable resin composition which exhibits excellent flow properties during molding, which will neither stain the metal mold nor exude onto the surface of the cured material, has an excellent mold-releasability, and converts to a cured material having excellent flexibility, a low thermal expansion coefficient, and a low mold shrinkage ratio.